Re: negative streamer propagation?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:

>Original poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
>Tesla list wrote:
>  >
>  > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
>  > "Spark Discharge", E. M. Bazalyan, Yu. P. Raizer, CRC Press, 1997, ISBN
>  > 0849328683 - See Chapter 6 (The Leader Process) for quantitative 
> discussion
>  > of positive leader mechanism, and section 6.11.1 (A Negative Leader), 
> pages
>  > 353 - 256 for qualitative discussion of why long gap breakdown threshold
>  > for negative discharges is significantly higher than for positive 
> discharges.
>In static machines it's easy to observe than if the spark gap is made
>with balls of different sizes, long sparks are only obtained if the
>smaller ball is positive. If the smaller ball is negative, the result
>is just corona at the negative terminal.
>So, I like to think that the breakdown voltage for -negative- polarity
>is that is smaller, but negative breakdown only tends to cause uniform
>corona, and not streamers/sparks. The positive terminal must be smaller
>to cause a -long- spark to start there, or, more probably, to end there,
>allowing it the "capture" a negative leader coming from the negative
>terminal.
>The reason for the difference is the larger mobility of electrons, but
>it's not easy to see exactly how the mechanism operates.
>This picture below shows bipolar corona over a film plate:
>http://www.coe.ufrj.br/~acmq/turpplat.jpg
>The positive is at the left side. Observe that the negative corona
>propagated more easily.
>Other pictures (Lichtemberg figures in film):
>Positive: http://www.coe.ufrj.br/~acmq/gray123.jpg
>Negtive: http://www.coe.ufrj.br/~acmq/gray124.jpg
>Antonio Carlos M. de Queiroz
>
>.

Antonio,

Your observations are consistent with impulse testing of long rod-plane 
gaps. They also appear to be consistent with initial breakout measurements 
made by Terry some time back which showed that corona/Trichel pulses 
develop at lower voltages during negative voltage excursions.

The reason for the polarity difference for long spark propagation appears 
to be that, in a non-uniform (divergent) E-field, any free electrons in the 
gap will be increasing accelerated as they move towards a positive 
electrode since they are moving into an increasing stronger (converging) 
E-field. This increases the probability that they'll gain sufficient energy 
to create secondary electrons as they collide and ionize neutral gas 
molecules, supporting further avalanche breakdowns and streamer growth. In 
the case of a negatively polarized electrode, free electrons will be 
repelled by the diverging E-field into a region with a lower E-field. This 
tends to reduce the odds that they'll gain sufficient energy to create 
secondary electrons, and choking off further streamer growth.

-- Bert --
-- 
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